Fundamental of Physics
(Ed. 7 or 8)
Authors:
Halliday, Resnick, and walker
1
21 Electric
El t i Charge
Ch
21--1 What is Physics
21
You are surrounded by devices that depend on the
physics of electromagnetism,
electromagnetism which is the combination
of electric and magnetic phenomena.
One of the best workers in Physics was Faraday,
Faraday a
truly gifted experimenter with a talent for physical
intuition and visualization. → [Faraday’s induction law]
In the mid-19th century, Maxwell put Faraday’s ideas
into mathematical form, introduced many new ideas of
hi own, and
his
d putt electromagnetism
l t
ti
on a strong
t
theoretical basis. → [Maxwell’s Equations]
2
21--2 Electric Charge
21
Electric charge is an intrinsic characteristic of the
fundamental particles making up those objects; that is, it is
a property that
h comes automatically
i ll with
i h those
h
particles
i l
wherever they exist. → 與生俱來的特性
The vastt amountt off charge
Th
h
i an everyday
in
d
object
bj t is
i
usually hidden because the object contains equal amounts
of the two kinds of charge:
g p
positive charge
g and negative
g
charge. With such an equality — or balance — of charge,
the object is said to be electrically neutral; that is, it
contains no net charge.
charge
If the two types of charge are not in balance, then there
charge We say that an object is charged to indicate
is a net charge.
that it has a charge imbalance, or net charge.
3
When a glass rod is rubbed with silk
(絲織品), the glass loses some of its “-”
charge and then has a small
unbalanced
b l
d “+”
“ ” charge.
h
When the plastic rod is rubbed with
f
fur
( 毛 皮 ),
) the
th plastic
l ti gains
i
a smallll
unbalanced “-” charge.
Charges with the same electrical sign
repel each other, and charges with
opposite electrical signs attract each
other.
4
21--3 C
21
Conductors
d t
and
d IInsulators
l t
Conductors
are
materials
through which charge can move
rather freely. (e.g. copper)
Nonconductors (insulators)
are materials through which
charge cannot move freely.
Semiconductors are materials
that are intermediate between
conductors
d
and
d insulators.
i
l
5
21--4 Coulomb’s Law
21
The electrostatic force of attraction or repulsion between
two point charges, having charges q1and q2 and being
separated
t d by
b a distance
di t
r, has
h the
th magnitude
it d
6
The
e co
constant
s a in Eq.
q 21-4 has
as the
e value
a ue
The quantity ε0, called the permittivity constant, is
7
Similarities between the gravitational force and the
electrostatic force
(1) Same form as that of Newton’s equation for the
gravitational force
((2)) Both obeyy the principle
p
p of superposition.
p p
(3) The shell theorem that we found so useful in our study of
gravitation has analogs in electrostatics:
8
9
10
Self study
Self-study
11
21--5 Charge is Quantized
21
We now know that fluids themselves, such as air and
water, are not continuous but are made up of atoms and
molecules; matter is discrete.
discrete
Experiment shows that “electrical fluid” is also not
continuous but is made up of multiples of a certain
elementary charge.
• Any q that can be detected can be written as
in which e, the elementary charge, has the approximate
value
12
The elementary charge e is one of the important constants
of nature.
— You often see phrases — such as “the charge on a
sphere,” “the amount of charge transferred,” and “the
charge carried by the electron” — that suggest that charge
is a substance. You should, however, keep in mind what is
intended: Particles are the substance and charge happens
to be one of their properties, just as mass is.
When a physical quantity
such as charge can have
only discrete values rather
than any value, we say that
the q
quantityy is q
quantized
(量子化).
13
21--6 Charge is Conserved
21
If you rub a glass rod with silk, a “+” charge appears on
the rod. Measurement shows that a “-” charge of equal
magnitude appears on the silk.
silk This suggests that rubbing
does not create charge but only transfers (轉移) it from
one body to another, upsetting ( 打 亂 ) the electrical
neutrality of each body during the process.
毀滅
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Problems
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